1. Field of the Invention
The present invention relates to lithography scanners, specifically correcting pupil asymmetry in the illumination beams of lithography scanners.
2. Background Art
Conventional lithography scanners include, among other things, an illumination system that produces a uniform intensity distribution of illumination, which is produced from a received laser beam. It is desirable that the resulting scan-integrated illumination beam profile be as uniform as possible and that any uniformity errors be kept as small as possible. This is because illumination uniformity influences the ability of a lithography scanner to produce uniform line widths across an entire exposure field. Illumination uniformity errors can significantly impact the quality of devices produced by the lithography system.
One example of a popular conventional lithography system is a step and scan system (sometimes referred to as a scanner). A step and scan system creates an illuminated slot narrower than one exposure field. The system then scans the reticle and wafer synchronously by the slot to expose each field on the wafer. This process is repeated. Because of the nature of the system's operation, radiation energy in the scan direction is integrated, and as a result dose on the photo-active coating on the substrate can be non-uniform. Non-uniformity in the dose causes printing errors and degraded device performance.
Some lithography systems use uniformity correction systems to make the scan-integrated intensity profile uniform. Some of these uniformity correction systems achieve this effect by inserting attenuating or opaque (collectively referred to herein as attenuating) elements into the illumination edge beam at or near a focal plane, located between a pupil and a focus plane. Asymmetrically inserting attenuating elements (opposed attenuating elements inserted non-equal distances) near a focal plane asymmetrically apodizes the illumination pupil, and vignettes the illumination pupil. Varying the asymmetric insertion along the non-scanning direction causes a variation in pupil asymmetry.
Modern lithography scanners typically use intensity measurements and sophisticated algorithms to determine the correct position for the uniformity correction attenuating elements. The attenuating elements are positioned with consideration of illumination uniformity, total scan-integrated intensity, and symmetric insertion (in an attempt to minimize the pupil asymmetry induced by the uniformity correction system). Lithography scanners typically include integrated optical metrology components capable of measuring the light distribution about the illumination pupil, from which pupil asymmetry may be computed. Future scanners may be used for double exposure techniques that create the need for extremely tight overlay specifications because overlay error can cause critical dimension non-uniformity, leading to poor performance and low yield in the manufactured semiconductor devices. Pupil asymmetry may cause a focus-coupled image translation which, combined with typical focus errors, may make the new tight overlay specifications impossible to achieve.
Lithography scanners often use illumination systems that have asymmetric illumination pupils and scan-integrated pupil asymmetry that varies in the non-scanning direction. Normal use of the uniformity correction system (attenuating elements inserted to flatten uniformity) causes further non-zero scan-integrated pupil asymmetry that varies in the non-scanning direction. A uniformity correction system that is misaligned with respect to the illumination beam will tend to insert attenuating elements asymmetrically.
Since the existing attenuating element positioning algorithms consider intensity measurements only and not pupilgrams, the algorithms are forced to assume the illumination pupil fill inherent to the illumination beam is symmetric. The uniformity correction system is only a source of pupil asymmetry, and not an actuator for pupil asymmetry compensation.
Any of the above sources of pupil asymmetry may make tight overlay specifications impossible to achieve. What is needed is a system and method for reducing the scan-integrated pupil asymmetry.